Methods and apparatuses for determining spatial distribution of an absolute phase of RF transmit field B1+ and/or RF receive field B1−； in an MRI system are described herein. An example method can include selecting a transmit coil for which to measure the absolute phase of the RF transmit field B1+, exciting nuclear spins in MR nuclei using at least two transmit configurations of the transmit coil, and detecting first and MR signals arising from exciting nuclear spins in MR nuclei using first and second transmit configurations, respectively. The method can also include acquiring first and second sets of complex k-space data from the first and second MR signals, respectively, and estimating an absolute phase B1+ map of the transmit coil using the first set of complex k-space data and the second set of complex k-space data.
